1. Field of the Invention
The invention relates to a quasi-optical gyrotron comprising
a) first means for generating an electron beam passing in the direction of an electron beam axis,
b) second means for generating a static magnetic field, which is aligned parallel to the electron beam axis and through which the electron beam is compressed and forced into gyration,
c) a quasi-optical resonator, which exhibits two mirrors arranged opposite to one another on a resonator axis aligned perpendicular to the electron beam axis, in which resonator an alternating electromagnetic field of given frequency is excited by the gyration of the electron beam, and
d) third means for coupling out electromagnetic radiation from the resonator.
2. Discussion of Background
A quasi-optical gyrotron of the type initially mentioned is known, for example, from the Patent CH-664045 or from the article "Das Gyrotron, Schlusselkomponente fr Hochleistungs-Mikrowellensender" (The gyrotron, key component for high-power microwave transmitters), H. G. Mathews, Minh Quang Tran, Brown Boveri Review 6-1987, pages 303 to 307. Such a gyrotron operates at frequencies of typically 150 GHz and above and is capable of generating radiant powers of a few 100 kW in continuous-wave operation.
The gyrotron is a high-power microwave tube for heating fusion plasmas. Since the current fusion installations are experimental installations, it is desirable for it to be possible to tune the frequency of the transmitter over a sizeable frequency range.
In the case of all previously known high-power gyrotrons having a resonator, the useful oscillation bandwidth is approximately 10-20%. In the case of sizeable deviations of the oscillation frequency from the optimum frequency, efficiency becomes extremely low.
One possibility of extending the frequency range of conventional, quasi-optical gyrotrons is the use of crossed resonators, as is proposed in Swiss patent application CH-1490/89. A principal advantage of the crossed resonators is the possibility of switching over from one frequency to double that frequency within a short period (of less than 1 sec). This is achieved when the resonator geometry is chosen such that the optimum oscillation range of the second resonator (for the same beam parameters) is exactly double the frequency of the first. There is also the possibility of choosing two independent frequencies. In this case, it is also necessary to change the magnetic field (field strength) as well as the resonator.
The solution with the crossed resonators is not, however, capable of covering a sufficiently wide frequency range.
Moreover, attempts have been made for some time to improve the efficiency of the gyrotron by means of so-called sheet-beam guns. A sheet-beam gun optimized for the quasi-optical gyrotron with its cylindrical symmetry is described, for example, in U.S. patent application Ser. No. 07/570,794. The advantage of such an electron gun consists in that the current density in the resonator is kept small in the nodal surfaces of the alternating electromagnetic field, so that the kinetic energy of the electrons is converted as completely as possible into radiant energy. However, it happens that in the case of a crossed resonator the sheet-beam gun cannot display its advantages, because of the different orientation of the nodal surfaces in the various resonators.